A re-evaluation of the "oncogenic" nature of Wnt/beta-catenin signaling in melanoma and other cancers

Targeted BRAF inhibition impacts survival in melanoma patients with high levels of Wnt/β-catenin signaling

Unprecedented clinical responses have been reported in advanced stage metastatic melanoma patients treated with targeted inhibitors of constitutively activated mutant BRAF, which is present in approximately half of all melanomas. We and others have previously observed an association of elevated nuclear β-catenin with improved survival in molecularly-unselected melanoma patients. This study sought to determine whether levels of Wnt/β-catenin signaling in melanoma tumors prior to treatment might predict patient responses to BRAF inhibitors (BRAFi). We performed automated quantification of β-catenin immunohistochemical expression in pretreatment BRAF-mutant tumors from 32 BRAFi-treated melanoma patients. Unexpectedly, patients with higher nuclear β-catenin in their tumors did not exhibit the survival advantage previously observed in molecularly-unselected melanoma patients who did not receive BRAFi. In cultured melanoma cells treated with long-term BRAFi, activation of Wnt/β-catenin signaling is markedly inhibited, coinciding with a loss of the enhancement of BRAFi-induced apoptosis by WNT3A observed in BRAFi-naïve cells. Together, these observations suggest that long-term treatment with BRAFi can impact the interaction between BRAF/MAPK and Wnt/β-catenin signaling to affect patient outcomes. Studies with larger patient cohorts are required to determine whether nuclear β-catenin expression correlates with clinical responses to BRAFi and to specific mechanisms of acquired resistance to BRAFi. Understanding these pathway interactions will be necessary to facilitate efforts to individualize therapies for melanoma patients.

Approaches that ascertain the role of dietary compounds in colonic cancer cells

Preventive approaches against cancer have not been fully developed and applied. For example, the incidence of some types of cancer, including colon cancer, is highly dependent upon lifestyle, and therefore, amenable to prevention. Among the lifestyle factors, diet strongly affects the incidence of colon cancer; however, there are no definitive dietary recommendations that protect against this malignancy. The association between diet-derived bioactives and development of colonic neoplasms will remain ill defined if we do not take into account: (1) the identity of the metabolites present in the colonic lumen; (2) their concentrations in the colon; and (3) the effect of the colonic contents on the function of individual bioactives. We review two approaches that address these questions: the use of fecal water and in vitro models of the human colon. Compared to treatment with individual diet-derived compounds, the exposure of colon cancer cells to samples from fecal water or human colon simulators mimics closer the in vitro conditions and allows for more reliable studies on the effects of diet on colon cancer development. The rationale and the advantages of these strategies are discussed from the perspective of a specific question on how to analyze the combined effect of two types of bioactives, butyrate and polyphenol metabolites, on colon cancer cells.

β-Catenin transcriptional activity is minimal in canine osteosarcoma and its targeted inhibition results in minimal changes to cell line behaviour

Canine osteosarcoma (OS) is an aggressive malignancy associated with poor outcomes. Therapeutic improvements are likely to develop from an improved understanding of signalling pathways contributing to OS development and progression. The Wnt signalling pathway is of interest for its role in osteoblast differentiation, its dysregulation in numerous cancer types, and the relative frequency of cytoplasmic accumulation of β-catenin in canine OS. This study aimed to determine the biological impact of inhibiting canonical Wnt signalling in canine OS, by utilizing either β-catenin siRNA or a dominant-negative T-cell factor (TCF) construct. There were no consistent, significant changes in cell line behaviour with either method compared to parental cell lines. Interestingly, β-catenin transcriptional activity was three-fold higher in normal canine primary osteoblasts compared to canine OS cell lines. These results suggest canonical Wnt signalling is minimally active in canine OS and its targeted inhibition is not a relevant therapeutic strategy.

Changes of PI3K/AKT/BCL2 signaling proteins in congenital Giant Nevi: melanocytes contribute to their increased survival and integrity

Congenital Giant Nevi (CGN) are rare melanocytic lesions with the potential to regress into malignant melanoma. Simultaneous up-regulation and cooperative interactions of signaling pathways are crucial events in the pathogenesis of melanocytes. Our study aimed to identify changes in the expression and activation of proteins controlling survival and/or apoptosis of the key signaling pathways PI3K/AKT/BCL2 and Wnt/β-catenin of CGN melanocytes. We applied a model of cultured melanocytes from paired CGN and normal appearing skin, and Western blot (WB) analyzed the expression and activation profile of survival and anti-apoptotic proteins of these signaling pathways, growth pattern, cell cycle and apoptosis. WB analysis demonstrated a significant higher expression level of activated AKT and of BCL2 proteins in the CGN melanocytes compared with paired melanocytes from normal appearing skin. A relative increase in the level of GSK3 and FOXO1 proteins, down stream targets of AKT, as well as of pβ-catenin was also detected in the CGN melanocytes compared with the controls. These changes were not affected by growth of the CGN melanocytes in reduced serum (starvation). Both cell populations shared a similar growth pattern, with no significant differences in the proportion of apoptotic cells and in cell cycle fractions. These data demonstrate for the first time, changes in signaling proteins of cultured CGN melanocytes. Further, suggesting that the changes in AKT/BCL2 signaling molecules might mediate growth and anti-apoptosis processes at least in part, thus increasing the survival potential of CGN melanocytes and maintaining their integrity.

Wnt signaling potentiates nevogenesis

Cellular senescence is a stable proliferation arrest associated with an altered secretory pathway (senescence-associated secretory phenotype). Cellular senescence is also a tumor suppressor mechanism, to which both proliferation arrest and senescence-associated secretory phenotype are thought to contribute. The melanocytes within benign human nevi are a paradigm for tumor-suppressive senescent cells in a premalignant neoplasm. Here a comparison of proliferating and senescent melanocytes and melanoma cell lines by RNA sequencing emphasizes the importance of senescence-associated proliferation arrest in suppression of transformation. Previous studies showed that activation of the Wnt signaling pathway can delay or bypass senescence. Consistent with this, we present evidence that repression of Wnt signaling contributes to melanocyte senescence in vitro. Surprisingly, Wnt signaling is active in many senescent human melanocytes in nevi, and this is linked to histological indicators of higher proliferative and malignant potential. In a mouse, activated Wnt signaling delays senescence-associated proliferation arrest to expand the population of senescent oncogene-expressing melanocytes. These results suggest that Wnt signaling can potentiate nevogenesis in vivo by delaying senescence. Further, we suggest that activated Wnt signaling in human nevi undermines senescence-mediated tumor suppression and enhances the probability of malignancy.

Activation of Wnt/β-catenin signaling increases apoptosis in melanoma cells treated with trail

While the TRAIL pathway represents a promising therapeutic target in melanoma, resistance to TRAIL-mediated apoptosis remains a barrier to its successful adoption. Since the Wnt/β-catenin pathway has been implicated in facilitating melanoma cell apoptosis, we investigated the effect of Wnt/β-catenin signaling on regulating the responses of melanoma cells to TRAIL. Co-treatment of melanoma cell lines with WNT3A-conditioned media and recombinant TRAIL significantly enhanced apoptosis compared to treatment with TRAIL alone. This apoptosis correlates with increased abundance of the pro-apoptotic proteins BCL2L11 and BBC3, and with decreased abundance of the anti-apoptotic regulator Mcl1. We then confirmed the involvement of the Wnt/β-catenin signaling pathway by demonstrating that siRNA-mediated knockdown of an intracellular β-catenin antagonist, AXIN1, or treating cells with an inhibitor of GSK-3 also enhanced melanoma cell sensitivity to TRAIL. These studies describe a novel regulation of TRAIL sensitivity in melanoma by Wnt/β-catenin signaling, and suggest that strategies to enhance Wnt/β-catenin signaling in combination with TRAIL agonists warrant further investigation.

Pharmacodynamic effects and mechanisms of resistance to vemurafenib in patients with metastatic melanoma

PURPOSE To assess pharmacodynamic effects and intrinsic and acquired resistance mechanisms of the BRAF inhibitor vemurafenib in BRAF(V600)-mutant melanoma, leading to an understanding of the mechanism of action of vemurafenib and ultimately to optimization of metastatic melanoma therapy. METHODS In the phase II clinical study NP22657 (BRIM-2), patients received oral doses of vemurafenib (960 mg twice per day). Serial biopsies were collected to study changes in mitogen-activated protein kinase (MAPK) signaling, cell-cycle progression, and factors causing intrinsic or acquired resistance by immunohistochemistry, DNA sequencing, or somatic mutation profiling. Results Vemurafenib inhibited MAPK signaling and cell-cycle progression. An association between the decrease in extracellular signal-related kinase (ERK) phosphorylation and objective response was observed in paired biopsies (n = 22; P = .013). Low expression of phosphatase and tensin homolog showed a modest association with lower response. Baseline mutations in MEK1(P124) coexisting with BRAF(V600) were noted in seven of 92 samples; their presence did not preclude objective tumor responses. Acquired resistance to vemurafenib associated with reactivation of MAPK signaling as observed by elevated ERK1/2 phosphorylation levels in progressive lesions and the appearance of secondary NRAS(Q61) mutations or MEK1(Q56P) or MEK1(E203K) mutations. These two activating MEK1 mutations had not previously been observed in vivo in biopsies of progressive melanoma tumors. CONCLUSION Vemurafenib inhibits tumor proliferation and oncogenic BRAF signaling through the MAPK pathway. Acquired resistance results primarily from MAPK reactivation driven by the appearance of secondary mutations in NRAS and MEK1 in subsets of patients. The data suggest that inhibition downstream of BRAF should help to overcome acquired resistance.

Multilevel omic data integration in cancer cell lines: advanced annotation and emergent properties

Background

High-throughput (omic) data have become more widespread in both quantity and frequency of use, thanks to technological advances, lower costs and higher precision. Consequently, computational scientists are confronted by two parallel challenges: on one side, the design of efficient methods to interpret each of these data in their own right (gene expression signatures, protein markers, etc.) and, on the other side, realization of a novel, pressing request from the biological field to design methodologies that allow for these data to be interpreted as a whole, i.e. not only as the union of relevant molecules in each of these layers, but as a complex molecular signature containing proteins, mRNAs and miRNAs, all of which must be directly associated in the results of analyses that are able to capture inter-layers connections and complexity.

Results

We address the latter of these two challenges by testing an integrated approach on a known cancer benchmark: the NCI-60 cell panel. Here, high-throughput screens for mRNA, miRNA and proteins are jointly analyzed using factor analysis, combined with linear discriminant analysis, to identify the molecular characteristics of cancer. Comparisons with separate (non-joint) analyses show that the proposed integrated approach can uncover deeper and more precise biological information. In particular, the integrated approach gives a more complete picture of the set of miRNAs identified and the Wnt pathway, which represents an important surrogate marker of melanoma progression. We further test the approach on a more challenging patient-dataset, for which we are able to identify clinically relevant markers.

Conclusions

The integration of multiple layers of omics can bring more information than analysis of single layers alone. Using and expanding the proposed integrated framework to integrate omic data from other molecular levels will allow researchers to uncover further systemic information. The application of this approach to a clinically challenging dataset shows its promising potential.

WNT7B mediates autocrine Wnt/β-catenin signaling and anchorage-independent growth in pancreatic adenocarcinoma

Developmental and cancer models show Wnt/β-catenin-dependent signaling mediates diverse phenotypic outcomes in the pancreas that are dictated by context, duration and strength of activation. While generally assumed to be pro-tumorigenic, it is unclear to what extent dysregulation of Wnt/β-catenin signaling impacts tumor progression in pancreatic adenocarcinoma (PDAC). In the present study, Wnt/β-catenin activity was characterized across a spectrum of PDAC cell lines and primary tumors. Reporter and gene expression based assays revealed wide heterogeneity in Wnt/β-catenin transcriptional activity across PDAC cell lines and patient tumors, as well as variable responsiveness to exogenous Wnt ligand stimulation. An experimentally-generated, pancreas-specific gene expression signature of Wnt/β-catenin transcriptional activation was used to stratify pathway activation across a cohort of resected, early stage PDAC tumors (N=41). In this cohort, higher Wnt/β-catenin activation was found to significantly correlate with lymphvascular invasion and worse disease specific survival (median survival time 20.3 versus 43.9 months, log rank P=0.03). Supporting the importance of Wnt ligand in mediating autocrine Wnt signaling, Wnt/β-catenin activity was significantly inhibited in PDAC cell lines by WLS gene silencing and the small molecule inhibitor IWP-2, both of which functionally block Wnt ligand processing and secretion. Transcriptional profiling revealed elevated expression of WNT7B occurred in PDAC cell lines with high levels of cell autonomous Wnt/β-catenin activity. Gene knockdown studies in AsPC-1 and HPAF-2 cell lines confirmed WNT7B mediated cell autonomous Wnt/β-catenin activation, as well as an anchorage-independent growth phenotype. Our findings indicate WNT7B can serve as a primary determinant of differential Wnt/β-catenin activation in PDAC. Disrupting the interaction between Wnt ligands and their receptors may be a particularly suitable approach for therapeutic modulation of Wnt/β-catenin signaling in PDAC and other cancer contexts where Wnt activation is mediated by ligand expression rather than mutations in canonical pathway members.

In vivo and in vitro models for the therapeutic targeting of Wnt signaling using a Tet-OΔN89β-catenin system

Although significant progress has been made in understanding the importance of Wnt signaling in the initiation of colorectal cancer, less is known about responses that accompany the reversal of oncogenic Wnt signaling. The aim of this study was to analyze in vivo and in vitro responses to an 'ideal' Wnt pathway inhibitor as a model for the therapeutic targeting of the pathway. A tetracycline-inducible transgenic mouse model expressing truncated β-catenin (ΔN89β-catenin) that exhibited a strong intestinal hyperplasia was analyzed during the removal of oncogenic β-catenin expression both in 3D 'crypt culture' and in vivo. Oncogenic Wnt signaling was rapidly and completely reversed. The strongest inhibition of Wnt target gene expression occurred within 24 h of doxycycline removal at which time the target genes Ascl2, Axin2 and C-myc were downregulated to levels below that in the control intestine. In vitro, the small molecule Wnt inhibitor CCT036477 induced a response within 4 h of treatment. By 7 days following doxycycline withdrawal, gene expression, cell proliferation and tissue morphology were undistinguishable from control animals.In conclusion, these results demonstrate that the reversal of Wnt signaling by inhibitors should ideally be studied within hours of treatment. The reversible system described, involving medium throughput in vitro approaches and rapid in vivo responses, should allow the rapid advance of early stage compounds into efficacy models that are more usually considered later in the drug discovery pipeline.

Nuclear LEF1/TCF4 correlate with poor prognosis but not with nuclear β-catenin in cerebral metastasis of lung adenocarcinomas

An essential function of the transcription factors LEF1/TCF4 in cerebral metastases of lung adenocarcinomas has been described in mouse models, suggesting a WNT/β-catenin effect as potential mechanism. Their role in humans is still unclear, thus we analyzed LEF1, TCF4, β-catenin, and early stage prognostic markers in 25 adenocarcinoma brain metastases using immunohistochemistry (IHC). IHC revealed nuclear TCF4 in all adenocarcinoma samples, whereas only 36 % depicted nuclear LEF1 and nuclear β-catenin signals. Samples with nuclear LEF1 as well as high TCF4 (++++) expression were associated with a shorter survival (p = 0.01, HR = 6.68), while nuclear β-catenin had no significant impact on prognosis and did not significantly correlate with nuclear LEF1. High proliferation index Ki67 was associated with shorter survival in late-stage disease (p = 0.03, HR 3.27). Additionally, we generated a LEF1/TCF4 as well as an AXIN2 signature, the latter as representative of WNT/β-catenin activity, following a bioinformatics approach with a gene expression dataset of cerebral metastases in lung adenocarcinoma. To analyze the prognostic relevance in primary lung adenocarcinomas, we applied both signatures to a microarray dataset of 58 primary lung adenocarcinomas. Only the LEF1/TCF4 signature was able to separate clusters with impact on survival (p = 0.01, HR = 0.32). These clusters displayed diverging enrichment patterns of the cell cycle pathway. In conclusion, our data show that LEF1/TCF4, but not β-catenin, have prognostic relevance in primary and cerebrally metastasized human lung adenocarcinomas. In contrast to the previous in vivo findings, these results indicate that LEF1/TCF4 act independently of β-catenin in this setting.

Conditional disruption of Axin1 leads to development of liver tumors in mice

BACKGROUND & AIMS

Mutations in components of the Wnt signaling pathway, including β-catenin and AXIN1, are found in more than 50% of human hepatocellular carcinomas (HCCs). Disruption of Axin1 causes embryonic lethality in mice. We generated mice with conditional disruption of Axin1 to study its function specifically in adult liver.

METHODS

Mice with a LoxP-flanked allele of Axin1 were generated by homologous recombination. Mice homozygous for the Axin1fl/fl allele were crossed with AhCre mice; in offspring, Axin1 was disrupted in liver following injection of β-naphthoflavone (Axin1fl/fl/Cre mice). Liver tissues were collected and analyzed by quantitative real-time polymerase chain reaction and immunoprecipitation, histology, and immunoblot assays.

RESULTS

Deletion of Axin1 from livers of adult mice resulted in an acute and persistent increase in hepatocyte cell volume, proliferation, and transcription of genes that induce the G(2)/M transition in the cell cycle and cytokinesis. A subset of Wnt target genes was activated, including Axin2, c-Myc, and cyclin D1. However, loss of Axin1 did not increase nuclear levels of β-catenin or cause changes in liver zonation that have been associated with loss of the adenomatous polyposis coli (APC) or constitutive activation of β-catenin. After 1 year, 5 of 9 Axin1fl/fl/Cre mice developed liver tumors with histologic features of HCC.

CONCLUSIONS

Hepatocytes from adult mice with conditional disruption of Axin1 in liver have a transcriptional profile that differs from that associated with loss of APC or constitutive activation of β-catenin. It might be similar to a proliferation profile observed in a subset of human HCCs with mutations in AXIN1. Axin1fl/fl mice could be a useful model of AXIN1-associated tumorigenesis and HCC.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Differential gene expression between skin and cervix induced by the E7 oncoprotein in a transgenic mouse model

HPV16 E7 oncoprotein expression in K14E7 transgenic mice induces cervical cancer after 6 months of treatment with the co-carcinogen 17β-estradiol. In untreated mice, E7 also induces skin tumors late in life albeit at low penetrance. These findings indicate that E7 alters cellular functions in cervix and skin so as to predispose these organs to tumorigenesis. Using microarrays, we determined the global genes expression profile in cervical and skin tissue of young adult K14E7 transgenic mice without estrogen treatment. In these tissues, the E7 oncoprotein altered the transcriptional pattern of genes involved in several biological processes including signal transduction, transport, metabolic process, cell adhesion, apoptosis, cell differentiation, immune response and inflammatory response. Among the E7-dysregulated genes were ones not previously known to be involved in cervical neoplasia including DMBT1, GLI1 and 17βHSD2 in cervix, as well as MMP2, 12, 14, 19 and 27 in skin.

Endothelial Wnt/β-catenin signaling inhibits glioma angiogenesis and normalizes tumor blood vessels by inducing PDGF-B expression

Wnt modulates glioma vascularization by regulating PDGF-B expression.

Endothelial Wnt/β-catenin signaling is necessary for angiogenesis of the central nervous system and blood–brain barrier (BBB) differentiation, but its relevance for glioma vascularization is unknown. In this study, we show that doxycycline-dependent Wnt1 expression in subcutaneous and intracranial mouse glioma models induced endothelial Wnt/β-catenin signaling and led to diminished tumor growth, reduced vascular density, and normalized vessels with increased mural cell attachment. These findings were corroborated in GL261 glioma cells intracranially transplanted in mice expressing dominant-active β-catenin specifically in the endothelium. Enforced endothelial β-catenin signaling restored BBB characteristics, whereas inhibition by Dkk1 (Dickkopf-1) had opposing effects. By overactivating the Wnt pathway, we induced the Wnt/β-catenin–Dll4/Notch signaling cascade in tumor endothelia, blocking an angiogenic and favoring a quiescent vascular phenotype, indicated by induction of stalk cell genes. We show that β-catenin transcriptional activity directly regulated endothelial expression of platelet-derived growth factor B (PDGF-B), leading to mural cell recruitment thereby contributing to vascular quiescence and barrier function. We propose that reinforced Wnt/β-catenin signaling leads to inhibition of angiogenesis with normalized and less permeable vessels, which might prove to be a valuable therapeutic target for antiangiogenic and edema glioma therapy.

Activation of the canonical Wnt/β-catenin signalling pathway is rare in canine malignant melanoma tissue and cell lines

Canine malignant melanoma is a highly aggressive tumour associated with a poor overall survival rate due to both local disease recurrence and its highly metastatic nature. Similar to advanced melanoma in man, canine oral melanoma is poorly responsive to conventional anti-cancer therapies. The lack of sustainable disease control warrants investigation of novel therapies, preferably targeting features specific to the tumour and different from normal cells. The Wnt signalling pathway is known to contribute to melanocytic lineage development in vertebrates and perturbation of the Wnt/β-catenin pathway has been implicated in numerous cancer types. Alterations of the Wnt/β-catenin pathway are suggested to occur in a subset of human melanomas, although the precise role of the Wnt/β-catenin pathway in melanoma is yet to be defined. This study investigates the activation status of the canonical Wnt/β-catenin pathway in canine malignant melanoma and its potential as a therapeutic target for treating this disease. The data indicate that canonical Wnt/β-catenin pathway activation is a rare event in canine oral malignant melanoma tissue and canine malignant melanoma cell lines.

GATA6 and FOXA2 regulate Wnt6 expression during extraembryonic endoderm formation

One of the earliest epithelial-to-mesenchymal transitions in mouse embryogenesis involves the differentiation of inner cell mass cells into primitive and then into parietal endoderm. These processes can be recapitulated in vitro using F9 teratocarcinoma cells, which differentiate into primitive endoderm when treated with retinoic acid (RA) and into parietal endoderm with subsequent treatment with dibutyryl cyclic adenosine monophosphate (db-cAMP). Our previous work on how primitive endoderm develops revealed that the Wnt6 gene is upregulated by RA, leading to the activation of the canonical WNT-β-catenin pathway. The mechanism by which Wnt6 is regulated was not determined, but in silico analysis of the human WNT6 promoter region had suggested that the GATA6 and FOXA2 transcription factors might be involved [1]. Subsequent analysis determined that both Gata6 and Foxa2 mRNA are upregulated in F9 cells treated with RA or RA and db-cAMP. More specifically, overexpression of Gata6 or Foxa2 alone induced molecular and morphological markers of primitive endoderm, which occurred concomitantly with the upregulation of the Wnt6 gene. Gata6- or Foxa2-overexpressing cells were also found to have increased levels in T-cell factor (TCF)-dependent transcription, and when these cells were treated with db-cAMP, they developed into parietal endoderm. Chromatin immunoprecipitation analysis revealed that GATA6 and FOXA2 were bound to the Wnt6 promoter, and overexpression studies showed that these transcription factors were sufficient to switch on the gene expression of a Wnt6 reporter construct. Together, these results provide evidence for the direct regulation of Wnt6 that leads to the activation of the canonical WNT-β-catenin pathway and subsequent induction of primitive extraembryonic endoderm.

Wnt/β-catenin signalling in prostate cancer

The Wnts are secreted cysteine-rich glycoproteins that have important roles in the developing embryo as well as in tissue homeostasis in adults. Dysregulation of Wnt signalling can lead to several types of cancer, including prostate cancer. A hallmark of the signalling pathway is the stabilization of the transcriptional co-activator β-catenin, which not only regulates expression of many genes implicated in cancer but is also an essential component of cadherin cell adhesion complexes. β-catenin regulates gene expression by binding members of the T-cell-specific transcription factor/lymphoid enhancer-binding factor 1 (TCF/LEF-1) family of transcription factors. In addition, β-catenin associates with the androgen receptor, a key regulator of prostate growth that drives prostate cancer progression. Wnt/β-catenin signalling can be controlled by secreted Wnt antagonists, many of which are downregulated in cancer. Activation of the Wnt/β-catenin pathway has effects on prostate cell proliferation, differentiation and the epithelial-mesenchymal transition, which is thought to regulate the invasive behaviour of tumour cells. However, whether targeting Wnt/β-catenin signalling is a good therapeutic option for prostate cancer remains unclear.

The many faces and functions of β-catenin

β-Catenin (Armadillo in Drosophila) is a multitasking and evolutionary conserved molecule that in metazoans exerts a crucial role in a multitude of developmental and homeostatic processes. More specifically, β-catenin is an integral structural component of cadherin-based adherens junctions, and the key nuclear effector of canonical Wnt signalling in the nucleus. Imbalance in the structural and signalling properties of β-catenin often results in disease and deregulated growth connected to cancer and metastasis. Intense research into the life of β-catenin has revealed a complex picture. Here, we try to capture the state of the art: we try to summarize and make some sense of the processes that regulate β-catenin, as well as the plethora of β-catenin binding partners. One focus will be the interaction of β-catenin with different transcription factors and the potential implications of these interactions for direct cross-talk between β-catenin and non-Wnt signalling pathways.

Calpain induces N-terminal truncation of β-catenin in normal murine liver development: diagnostic implications in hepatoblastomas

Hepatic competence, specification, and liver bud expansion during development depend on precise temporal modulation of the Wnt/β-catenin signaling. Also, loss- and gain-of-function studies have revealed pleiotropic roles of β-catenin in proliferation and hepatocyte and biliary epithelial cell differentiation, but precise mechanisms remain unknown. Here we utilize livers from different stages of murine development to determine β-catenin signaling and downstream targets. Although during early liver development full-length β-catenin is the predominant form, at late stages, where full-length β-catenin localizes to developing biliary epithelial cells only, a 75-kDa truncated β-catenin species is the principal form localizing at the membrane and in the nucleus of differentiating hepatocytes. The truncated species lacks 95 N-terminal amino acids and is transcriptionally active. Our evidence points to proteolytic cleavage of β-catenin by calpain as the mechanism of truncation in cell-free and cell-based assays. Intraperitoneal injection of a short term calpain inhibitor to timed pregnant female mice abrogated β-catenin truncation in the embryonic livers. RNA-seq revealed a unique set of targets transcribed in cells expressing truncated versus full-length β-catenin, consistent with different functionalities. A further investigation using N- and C-terminal-specific β-catenin antibodies on human hepatoblastomas revealed a correlation between full-length versus truncated β-catenin and differentiation status, with embryonal hepatoblastomas expressing full-length β-catenin and fetal hepatoblastomas expressing β-catenin lacking its N terminus. Thus we conclude that calpain-mediated cleavage of β-catenin plays a role in regulating hepatoblast differentiation in mouse and human liver, and the presence of the β-catenin N terminus correlates with differentiation status in hepatoblastomas.

Why are epididymal tumours so rare?

Epididymal tumour incidence is at most 0.03% of all male cancers. It is an enigma why the human epididymis does not often succumb to cancer, when it expresses markers of stem and cancer cells, and constitutively expresses oncogenes, pro-proliferative and pro-angiogenic factors that allow tumour cells to escape immunosurveillance in cancer-prone tissues. The privileged position of the human epididymis in evading tumourigenicity is reflected in transgenic mouse models in which induction of tumours in other organs is not accompanied by epididymal neoplasia. The epididymis appears to: (i) prevent tumour initiation (it probably lacks stem cells and has strong anti-oxidative mechanisms, active tumour suppressors and inactive oncogene products); (ii) foster tumour monitoring and destruction (by strong immuno-surveillance and -eradication, and cellular senescence); (iii) avert proliferation and angiogenesis (with persistent tight junctions, the presence of anti-angiogenic factors and misplaced pro-angiogenic factors), which together (iv) promote dormancy and restrict dividing cells to hyperplasia. Epididymal cells may be rendered non-responsive to oncogenic stimuli by the constitutive expression of factors generally inducible in tumours, and resistant to the normal epididymal environment, which mimics that of a tumour niche promoting tumour growth. The threshold for tumour initiation may thus be higher in the epididymis than in other organs. Several anti-tumour mechanisms are those that maintain spermatozoa quiescent and immunologically silent, so the low incidence of cancer in the epididymis may be a consequence of its role in sperm maturation and storage. Understanding these mechanisms may throw light on cancer prevention and therapy in general.

Dysregulation of Wnt/β-catenin signaling in gastrointestinal cancers

Aberrant Wnt/β-catenin signaling is widely implicated in numerous malignancies, including cancers of the gastrointestinal tract. Dysregulation of signaling is traditionally attributed to mutations in Axin, adenomatous polyposis coli, and β-catenin that lead to constitutive hyperactivation of the pathway. However, Wnt/β-catenin signaling is also modulated through various other mechanisms in cancer, including cross talk with other altered signaling pathways. A more complex view of Wnt/β-catenin signaling and its role in gastrointestinal cancers is now emerging as divergent phenotypic outcomes are found to be dictated by temporospatial context and relative levels of pathway activation. This review summarizes the dysregulation of Wnt/β-catenin signaling in colorectal carcinoma, hepatocellular carcinoma, and pancreatic ductal adenocarcinoma, with particular emphasis on the latter two. We conclude by addressing some of the major challenges faced in attempting to target the pathway in the clinic.

DNAJB6 chaperones PP2A mediated dephosphorylation of GSK3β to downregulate β-catenin transcription target, osteopontin

Elevated levels of the oncoprotein, osteopontin (OPN), are associated with poor outcome of several types of cancers including melanoma. We have previously reported an important involvement of DNAJB6, a member of heat-shock protein 40 (HSP40) family, in negatively impacting tumor growth. The current study was prompted by our observations reported here which revealed a reciprocal relationship between DNAJB6 and OPN in melanoma specimens. The 'J domain' is the most conserved domain of HSP40 family of proteins. Hence, we assessed the functional role of the J domain in activities of DNAJB6. We report that the J domain of DNAJB6 is involved in mediating OPN suppression. Deletion of the J domain renders DNAJB6 incapable of impeding malignancy and suppressing OPN. Our mechanistic investigations reveal that DNAJB6 binds HSPA8 (heat-shock cognate protein, HSC70) and causes dephosphorylation of glycogen synthase kinase 3β (GSK3β) at Ser 9 by recruiting protein phosphatase, PP2A. This dephosphorylation activates GSK3β, leading to degradation of β-catenin and subsequent loss of TCF/LEF (T cell factor1/lymphoid enhancer factor1) activity. Deletion of the J domain abrogates assembly of this multiprotein complex and renders GSK3β inactive, thus, stabilizing β-catenin, a transcription co-activator for OPN expression. Our in-vitro and in-vivo functional analyses show that silencing OPN expression in the background of deletion of the J domain renders the resultant tumor cells less malignant despite the presence of stabilized β-catenin. Thus, we have uncovered a new mechanism for regulation of GSK3β activity leading to inhibition of Wnt/β-catenin signaling.

DNAJB6 chaperones PP2A mediated dephosphorylation of GSK3β to downregulate β-catenin transcription target, osteopontin

Elevated levels of the oncoprotein, osteopontin (OPN), are associated with poor outcome of several types of cancers including melanoma. We have previously reported an important involvement of DNAJB6, a member of heat-shock protein 40 (HSP40) family, in negatively impacting tumor growth. The current study was prompted by our observations reported here which revealed a reciprocal relationship between DNAJB6 and OPN in melanoma specimens. The 'J domain' is the most conserved domain of HSP40 family of proteins. Hence, we assessed the functional role of the J domain in activities of DNAJB6. We report that the J domain of DNAJB6 is involved in mediating OPN suppression. Deletion of the J domain renders DNAJB6 incapable of impeding malignancy and suppressing OPN. Our mechanistic investigations reveal that DNAJB6 binds HSPA8 (heat-shock cognate protein, HSC70) and causes dephosphorylation of glycogen synthase kinase 3β (GSK3β) at Ser 9 by recruiting protein phosphatase, PP2A. This dephosphorylation activates GSK3β, leading to degradation of β-catenin and subsequent loss of TCF/LEF (T cell factor1/lymphoid enhancer factor1) activity. Deletion of the J domain abrogates assembly of this multiprotein complex and renders GSK3β inactive, thus, stabilizing β-catenin, a transcription co-activator for OPN expression. Our in-vitro and in-vivo functional analyses show that silencing OPN expression in the background of deletion of the J domain renders the resultant tumor cells less malignant despite the presence of stabilized β-catenin. Thus, we have uncovered a new mechanism for regulation of GSK3β activity leading to inhibition of Wnt/β-catenin signaling.

Wnt/β-catenin signaling and AXIN1 regulate apoptosis triggered by inhibition of the mutant kinase BRAFV600E in human melanoma

Because the Wnt/β-catenin signaling pathway is linked to melanoma pathogenesis and to patient survival, we conducted a kinome small interfering RNA (siRNA) screen in melanoma cells to expand our understanding of the kinases that regulate this pathway. We found that BRAF signaling, which is constitutively activated in many melanomas by the BRAF(V600E) mutation, inhibits Wnt/β-catenin signaling in human melanoma cells. Because inhibitors of BRAF(V600E) show promise in ongoing clinical trials, we investigated whether altering Wnt/β-catenin signaling might enhance the efficacy of the BRAF(V600E) inhibitor PLX4720. We found that endogenous β-catenin was required for PLX4720-induced apoptosis of melanoma cells and that activation of Wnt/β-catenin signaling synergized with PLX4720 to decrease tumor growth in vivo and to increase apoptosis in vitro. This synergistic enhancement of apoptosis correlated with reduced abundance of an endogenous negative regulator of β-catenin, AXIN1. In support of the hypothesis that AXIN1 is a mediator rather than a marker of apoptosis, siRNA directed against AXIN1 rendered resistant melanoma cell lines susceptible to apoptosis in response to treatment with a BRAF(V600E) inhibitor. Thus, Wnt/β-catenin signaling and AXIN1 may regulate the efficacy of inhibitors of BRAF(V600E), suggesting that manipulation of the Wnt/β-catenin pathway could be combined with BRAF inhibitors to treat melanoma.

How do they do Wnt they do?: regulation of transcription by the Wnt/β-catenin pathway

Wnt/β-catenin signalling is known to play many roles in metazoan development and tissue homeostasis. Misregulation of the pathway has also been linked to many human diseases. In this review, specific aspects of the pathway's involvement in these processes are discussed, with an emphasis on how Wnt/β-catenin signalling regulates gene expression in a cell and temporally specific manner. The T-cell factor (TCF) family of transcription factors, which mediate a large portion of Wnt/β-catenin signalling, will be discussed in detail. Invertebrates contain a single TCF gene that contains two DNA-binding domains, the high mobility group (HMG) domain and the C-clamp, which increases the specificity of DNA binding. In vertebrates, the situation is more complex, with four TCF genes producing many isoforms that contain the HMG domain, but only some of which possess a C-clamp. Vertebrate TCFs have been reported to act in concert with many other transcription factors, which may explain how they obtain sufficient specificity for specific DNA sequences, as well as how they achieve a wide diversity of transcriptional outputs in different cells.

A switch from canonical to noncanonical Wnt signaling mediates drug resistance in colon cancer cells

Butyrate, a fermentation product of fiber in the colon, acts as a histone deacetylase inhibitor (HDACi) and induces apoptosis in colon cancer (CC) cells in vitro. We have reported that the apoptotic effects of butyrate are dependent upon the hyperactivation of the Wnt/beta-catenin pathway. However, prolonged exposure of CC cells to increasing concentrations of butyrate results in the acquisition of resistance to the Wnt/beta-catenin- and apoptosis-inducing effects of this agent, as well as cross-resistance to structurally different HDACis. Here we report that one mechanism whereby HDACi resistance arises is through the increase of beta-catenin-independent (noncanonical) Wnt signaling. Compared to HDACi-sensitive HCT-116 CC cells, HDACi-resistant HCT-R cells exhibit higher levels of AKT/PKB cell survival signaling, which is in part induced by WNT5A and its receptor ROR2. The induction of AKT signaling by HDACis is also detected in other CC cell lines, albeit to a lesser extent than in the drug-resistant HCT-R cells. The observations suggested that the apoptotic effect of butyrate and other HDACis in CC cells can be augmented by inhibitors of pAKT. In agreement with the hypothesis, the combination of MK2206, a pAKT inhibitor, and a HDACi (butyrate or LBH589) induced higher apoptosis in CC cells compared to each agent alone. The exposure to both agents also re-sensitized the HCT-R cells to apoptosis. Finally, the concept of simultaneously inducing canonical Wnt activity and suppressing AKT signaling was translated into a combination of diet-derived agents. Diet-derived pAKT inhibitors (caffeic acid phethyl ester, sulforaphane, dilallyl trisulfide) suppressed the butyrate-induced levels of pAKT, and increased the apoptotic effects of butyrate in both drug-sensitive and drug-resistant CC cells.

Our findings can be translated into (a) CC therapy employing combinations of synthetic HDACis and inhibitors of pAKT, as well as (b) CC prevention based upon diets that result in sufficient amounts of butyrate and pAKT inhibitors.

The gold (III) porphyrin complex, gold-2a, suppresses WNT1 expression in breast cancer cells by enhancing the promoter association of YY1

The gold (III) porphyrin complex, gold-2a, elicits anti-tumor activity by targeting the Wnt/β-catenin signaling pathway [Chow KH et al, Cancer Research 2010;70(1):329-37]. Here, the molecular mechanisms underlying the inhibitory effects of this compound on WNT1 gene expression were elucidated further. A response element to gold-2a was identified located within the -1290 to -1112 nt region of the WNT1 promoter, containing a binding site for the transcription regulator Yin Yang 1 (YY1). Gold-2a promoted the association of YY1 and suppressor of zeste 12 (Suz12; a component of the polycomb repressor complex 2) with the WNT1 promoter. Under normal culture conditions, the intracellular translocalization of YY1 was synchronized with cell cycle progression and WNT1 expression. Gold-2a promoted the nuclear accumulation and abolished the nuclear exportation of YY1, resulting in a persistent inhibition of WNT1 expression and a cell cycle arrest at G1/S phase. A dimorphic role of YY1 in regulating cell proliferation and division was revealed. Thus, the present study extends the understanding of the anti-tumor mechanism of gold-2a to the epigenetic level, which involves the modulation of the dynamic interactions between YY1 and a specific region of the WNT1 promoter.

Silencing of the Wnt transcription factor TCF4 sensitizes colorectal cancer cells to (chemo-) radiotherapy

A considerable percentage of rectal cancers are resistant to standard preoperative chemoradiotherapy. Because patients with a priori-resistant tumors do not benefit from multimodal treatment, understanding and overcoming this resistance remains of utmost clinical importance. We recently reported overexpression of the Wnt transcription factor TCF4, also known as TCF7L2, in rectal cancers that were resistant to 5-fluorouracil-based chemoradiotherapy. Because Wnt signaling has not been associated with treatment response, we aimed to investigate whether TCF4 mediates chemoradioresistance. RNA interference-mediated silencing of TCF4 was employed in three colorectal cancer (CRC) cell lines, and sensitivity to (chemo-) radiotherapy was assessed using a standard colony formation assay. Silencing of TCF4 caused a significant sensitization of CRC cells to clinically relevant doses of X-rays. This effect was restricted to tumor cells with high T cell factor (TCF) reporter activity, presumably in a β-catenin-independent manner. Radiosensitization was the consequence of (i) a transcriptional deregulation of Wnt/TCF4 target genes, (ii) a silencing-induced G(2)/M phase arrest, (iii) an impaired ability to adequately halt cell cycle progression after radiation and (iv) a compromised DNA double strand break repair as assessed by γH2AX staining. Taken together, our results indicate a novel mechanism through which the Wnt transcription factor TCF4 mediates chemoradioresistance. Moreover, they suggest that TCF4 is a promising molecular target to sensitize resistant tumor cells to (chemo-) radiotherapy.

β-Catenin signaling increases during melanoma progression and promotes tumor cell survival and chemoresistance

Beta-catenin plays an important role in embryogenesis and carcinogenesis by controlling either cadherin-mediated cell adhesion or transcriptional activation of target gene expression. In many types of cancers nuclear translocation of beta-catenin has been observed. Our data indicate that during melanoma progression an increased dependency on the transcriptional function of beta-catenin takes place. Blockade of beta-catenin in metastatic melanoma cell lines efficiently induces apoptosis, inhibits proliferation, migration and invasion in monolayer and 3-dimensional skin reconstructs and decreases chemoresistance. In addition, subcutaneous melanoma growth in SCID mice was almost completely inhibited by an inducible beta-catenin knockdown. In contrast, the survival of benign melanocytes and primary melanoma cell lines was less affected by beta-catenin depletion. However, enhanced expression of beta-catenin in primary melanoma cell lines increased invasive capacity in vitro and tumor growth in the SCID mouse model. These data suggest that beta-catenin is an essential survival factor for metastatic melanoma cells, whereas it is dispensable for the survival of benign melanocytes and primary, non-invasive melanoma cells. Furthermore, beta-catenin increases tumorigenicity of primary melanoma cell lines. The differential requirements for beta-catenin signaling in aggressive melanoma versus benign melanocytic cells make beta-catenin a possible new target in melanoma therapy.

β-catenin as a potential key target for tumor suppression

β-catenin is a multifunctional protein identified to be pivotal in embryonic patterning, organogenesis and adult homeostasis. It plays a critical structural role in mediating cadherin junctions and is also an essential transcriptional co-activator in the canonical Wnt pathway. Evidence has been documented that both the canonical Wnt pathway and cadherin junctions are deregulated or impaired in a plethora of human malignancies. In the light of this, there has been a recent surge in elucidating the mechanisms underlying the etiology of cancer development from the perspective of β-catenin. Here, we focus on the emerging roles of β-catenin in the process of tumorigenesis by discussing novel functions of old players and new proteins, mechanisms identified to mediate or interact with β-catenin and the most recently unraveled clinical implications of β-catenin regulatory pathways toward tumor suppression.

Modulation of Wnt/β-catenin signaling and proliferation by a ferrous iron chelator with therapeutic efficacy in genetically engineered mouse models of cancer

Using a screen for Wnt/β-catenin inhibitors, a family of 8-hydroxyquinolone derivatives with in vivo anti-cancer properties was identified. Analysis of microarray data for the lead compound N-((8-hydroxy-7-quinolinyl) (4-methylphenyl)methyl)benzamide (HQBA) using the Connectivity Map database suggested that it is an iron chelator that mimics the hypoxic response. HQBA chelates Fe²⁺ with a dissociation constant of ∼10⁻¹⁹ , with much weaker binding to Fe³⁺ and other transition metals. HQBA inhibited proliferation of multiple cell lines in culture, and blocked the progression of established spontaneous cancers in two distinct genetically engineered mouse models of mammary cancer, MMTV-Wnt1 and MMTV-PyMT mice, without overt toxicity. HQBA may inhibit an iron-dependent factor that regulates cell-type-specific β-catenin-driven transcription. It inhibits cancer cell proliferation independently of its effect on β-catenin signaling, as it works equally well in MMTV-PyMT tumors and diverse β-catenin-independent cell lines. HQBA is a promising specific intracellular Fe²⁺ chelator with activity against spontaneous mouse mammary cancers.

Network-guided analysis of genes with altered somatic copy number and gene expression reveals pathways commonly perturbed in metastatic melanoma

Cancer genomes frequently contain somatic copy number alterations (SCNA) that can significantly perturb the expression level of affected genes and thus disrupt pathways controlling normal growth. In melanoma, many studies have focussed on the copy number and gene expression levels of the BRAF, PTEN and MITF genes, but little has been done to identify new genes using these parameters at the genome-wide scale. Using karyotyping, SNP and CGH arrays, and RNA-seq, we have identified SCNA affecting gene expression ('SCNA-genes') in seven human metastatic melanoma cell lines. We showed that the combination of these techniques is useful to identify candidate genes potentially involved in tumorigenesis. Since few of these alterations were recurrent across our samples, we used a protein network-guided approach to determine whether any pathways were enriched in SCNA-genes in one or more samples. From this unbiased genome-wide analysis, we identified 28 significantly enriched pathway modules. Comparison with two large, independent melanoma SCNA datasets showed less than 10% overlap at the individual gene level, but network-guided analysis revealed 66% shared pathways, including all but three of the pathways identified in our data. Frequently altered pathways included WNT, cadherin signalling, angiogenesis and melanogenesis. Additionally, our results emphasize the potential of the EPHA3 and FRS2 gene products, involved in angiogenesis and migration, as possible therapeutic targets in melanoma. Our study demonstrates the utility of network-guided approaches, for both large and small datasets, to identify pathways recurrently perturbed in cancer.

Wnt signaling signaling at and above the receptor level

Wnt signaling is one of the most important developmental signaling pathways that controls cell fate decisions and tissue patterning during early embryonic and later development. It is activated by highly conserved Wnt proteins that are secreted as palmitoylated glycoproteins and act as morphogens to form a concentration gradient across a developing tissue. Wnt proteins regulate transcriptional and posttranscriptional processes depending on the distance of their origin and activate distinct intracellular cascades, commonly referred to as canonical (β-catenin-dependent) and noncanonical (β-catenin-independent) pathways. Therefore, the secretion and the diffusion of Wnt proteins needs to be tightly regulated to induce short- and long-range downstream signaling. Even though the Wnt signaling cascade has been studied intensively, key aspects and principle mechanisms, such as transport of Wnt growth factors or regulation of signaling specificity between different Wnt pathways, remain unresolved. Here, we introduce basic principles of Wnt/Wg signal transduction and highlight recent discoveries, such as the involvement of vacuolar ATPases and vesicular acidification in Wnt signaling. We also discuss recent findings regarding posttranslational modifications of Wnts, trafficking through the secretory pathway and developmental consequences of impaired Wnt secretion. Understanding the detailed mechanism and regulation of Wnt protein secretion will provide valuable insights into many human diseases based on overactivated Wnt signaling.

The Wnt/β-catenin signaling pathway in liver biology and disease

The Wnt signaling pathway is an evolutionarily conserved, highly complex signaling pathway that is critical for development, differentiation and cellular homeostasis. The protein β-catenin is the central player in one major arm of the Wnt pathway called the canonical Wnt pathway. As in other organs, the Wnt/β-catenin pathway is critical for liver development. However, recent research suggests that the pathway is also important in liver regeneration, liver metabolism and maintenance of normal function in the adult liver. Aberrant activation of β-catenin has also been implicated in the pathogenesis of hepatobiliary neoplasia, ranging from benign lesions to liver cancer. The explosion of research into the many roles of the Wnt/β-catenin pathway promises to change our fundamental understanding of normal liver biology and the aberrations that lead to disease and cancer.

Renin, (pro)renin and receptor: an update

PRR [(pro)renin receptor] was named after its biological characteristics, namely the binding of renin and of its inactive precursor prorenin, that triggers intracellular signalling involving ERK (extracellular-signal-regulated kinase) 1/2. However the gene encoding for PRR is named ATP6ap2 (ATPase 6 accessory protein 2) because PRR was initially found as a truncated form co-purifying with V-ATPase (vacuolar H+-ATPase). There are now data showing that this interaction is not only physical, but also functional in the kidney and the heart. However, the newest and most fascinating development of PRR is its involvement in both the canonical Wnt/β-catenin and non-canonical Wnt/PCP (planar cell polarity) pathways, which are essential for adult and embryonic stem cell biology, embryonic development and disease, including cancer. In the Wnt/β-catenin pathway, it has been shown that PRR acts as an adaptor between the Wnt receptor LRP5/6 (low-density lipoprotein receptor-related protein 5/6) and Fz (frizzled) and that the proton gradient generated by the V-ATPase in endosomes is necessary for LRP5/6 phosphorylation and β-catenin activation. In the Wnt/PCP pathway, PRR binds to Fz and controls its asymetrical subcellular distribution and therefore the polarization of the cells in a plane of a tissue. These essential cellular functions of PRR are independent of renin and open new avenues on the pathophysiological role of PRR. The present review will summarize our knowledge of (pro)renin-dependent functions of PRR and will discuss the newly recognized functions of PRR related to the V-ATPase and to Wnt signalling.